How to convert from high poly to low poly
One way to reduce mesh density is the decimate modifier, however for the workflow you want (creating a game asset) I think you want Retopology. 

This is a workflow which gives your more control over the final result, however it will take longer as it is a manual process.

Some helpful addons and built in functionality for retopologizing your mesh:


Surface Snapping (feature)
Shrinkwrap modifier (feature)
Bsurfaces (bundled addon)
RetopoFlow (addon)


Some tutorials and reference material:


Bsurfaces demo
wiki page on retopologizing


After you have a lower topology mesh you can bake normal maps etc. to make it appear more detailed than it really is.